1. Field of the Invention
This invention relates to a circuit arrangement for charging rechargeable batteries, having a first terminal and a second terminal to which at least one battery is connectable in order to be charged, and comprising a charging current source connected to both terminals and adapted to supply charging current pulse trains, which source has a control input to which a pulse-shaped control signal can be applied to enable and to inhibit the supply of a charging current pulse train, and which in accordance with the pulse-shaped control signal applied to the control input supplies to the at least one battery a charging current pulse train of charging current pulses which occur during successive charging periods which are spaced from one another by charging pauses.
2. Description of the Related Art
A circuit arrangement of the type defined in the opening paragraph is known, for example, from the international Patent Application published under publication number WO 92/22120.
Such a circuit arrangement serves for charging a battery as rapidly as possible but so as to minimise the rise in temperature of the battery and the increase in gas pressure in the battery during charging, because an excessive temperature rise and an excessive increase in gas pressure adversely affect the life and rechargeability of the battery and may even result in the battery being damaged or destroyed. During charging of a battery the electric energy applied to the battery brings about a change in chemical states in the battery in order to store the energy. The conversion of electric energy into different chemical states is a continuous process, which can be optimised if the supply of electric energy to a battery also proceeds as continuously as possible.
With the circuit arrangement known from WO 92/22120 the charging current is applied to a battery in the form of a train of charging current pulses of constant duration and constant amplitude, the charging current pulses being spaced from one another by spacing intervals which are variable in steps. The charging current pulses can be applied to a battery with pulse amplitudes which differ in steps but which are constant in each step, the instantaneous pulse amplitude in a step being dependent on the instantaneous charging condition of the battery. The charging condition of a battery is determined in the pulse spacings, the charging condition of the battery in each pulse spacing being determined by measuring the battery no-load voltage, which increases up to a maximum value as the charging process proceeds and subsequently decreases as the charging process proceeds after the maximum value has been reached, and the relevant measurement value is stored. The charging current pulses are given a pulse amplitude in accordance with the stored measurement value, which pulse amplitude has been assigned to the measured charging condition and is read from a memory. The choice of the applied pulse amplitude of the charging current pulses does not allow for the internal resistance of a battery, which depends on the age, condition, construction and storage capacity of a battery, which may lead to increased current loads of a battery when excessive pulse amplitudes are selected for the charging current pulses. The charging current pulses are separated from one another by pulse spacings whose spacing intervals, as already stated hereinbefore, can only be varied in steps, which means that the duty cycle of the charging current pulse trains can be changed only in steps. Thus, in the known circuit arrangement both the pulse amplitude of the charging current pulses and the duty cycle of the charging current pulse trains can be varied only in steps, so that the supply of electric energy to a battery to be charged can also be varied in steps only, as a result of which the process of convening electric energy into different chemical states in a battery, which is basically a continuous process, can only be an approximation and cannot be optimised with the electric energy thus applied to a battery. Moreover, since the supply of electric energy to a battery can be changed only in steps, adaptation of the charging process to the instantaneous charging condition of a battery can also be effected in steps only, so that it is not possible to obtain an optimum charging process which is continuously adapted to the instantaneous charging condition. Besides, the known circuit arrangement requires a measurement device suitable for determining the charging condition of a battery during the charging pauses, which device is generally formed by an expensive and intricate clocked measurement amplifier. In addition, storage devices are needed for storing both the measurement values of the battery no-load voltage determined in the relevant pulse spacings and the corresponding pulse amplitudes of the charging current pulses. Said measurement device as well as the storage devices are expensive and add to the cost of such a circuit arrangement.